Method and apparatus for flaw detection by leakage fluxes and leakage flux sensor

ABSTRACT

A leakage flux flaw detection apparatus in which an object material is magnetized and leakage fluxes derived from a flaw are detected to thereby search for a flaw of the object material in a predetermined flaw-detection direction. The apparatus has a pair of magnet poles for magnetizing the object. material in a direction different from the flaw-detection direction, another pair of magnet poles for magnetizing the object material in a direction different from the magnetization direction and flaw-detection direction of the first pair of magnet poles at a predetermined distance in the flaw-detection direction from the magnetization area of the two magnet poles, and two leakage sensors each of which is interposed between each of the two pairs of magnet poles. The leakage flux sensor which detects leakage fluxes from a flaw of the magnetized object material through two flux-sensing parts arranged in opposed relation to the object material and in predetermined spaced relation with each other along the flaw-detection direction includes two flux-sensing parts having the center distance d thereof not more than 4 mm, the length L of not less than 0.5 mm in the direction perpendicular to the flaw-detection direction, and the value d/L of not less than 1. The leakage flux sensor includes two concentrically-arranged flux-sensing parts having circular cross sections with different cross sectional areas.

This application is a 371 of PCT/JP95/01254 filed Jun. 22, 1995.

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for flawdetection in which an object material having magnetic properties ismagnetized and leakage fluxes generated due to a flaw, which aredetected to indicate a flaw of the object material, and a sensor fordetecting leakage fluxes generated due to a flaw of a magnetized objectmaterial having magnetic properties.

DESCRIPTION OF RELATED ART

A method for detecting a flaw in an object material having magneticproperties by magnetizing the object material and using leakage magneticfluxes generated due to a flaw such as a surface flaw or an internalflaw is well known.

FIG. 1 is a side view showing a model of a conventional leakage fluxflaw detection apparatus of a rotary type, and FIG. 2 is a diagramshowing a model partially enlarged from FIG,1. A couple of feeders 81,81 each including a pair of upper and lower guide rolls are arranged inpredetermined spaced relationship with each other in a transportationarea to hold and transport a solid-cylindrical or tubular objectmaterial P having ferromagnetic properties in the direction indicated byan arrow. The feeders 81, 81 have an annular rotary head 71 arrangedtherebetween in such a manner that the object material P is adapted topass through the rotary head 71. The rotary head 71 has mounted thereona pair of electromagnet poles 72, 72 in predetermined spacedrelationship with each other along the peripheral direction of therotary head 71 for magnetizing the object material P. A sensor 73 suchas a flux-sensing element or a search coil for detecting the changes ofmagnetic flux density due to a flaw is mounted at substantially thecenter between the electromagnet poles 72, 72.

The rotary head 71 is mounted coaxially with the center axis of anannular rotary unit 80 on the input side of the object material P fordriving the rotary head 71. The output signal of the sensor 73 of therotary head 71 is applied to a flaw signal analyzer 82 through therotary unit 80. A marking unit 83 for marking the position of any flawdetected is arranged on the output side of the rotary unit 80. Thismarking unit 83 attaches an appropriate mark on the surface of theobject material P in response to a command from the flaw signal analyzer82.

In this apparatus, the rotary head 71 is rotated by the rotary unit 80while the object material P is fed in the direction of the arrow bymeans of the feeders 81, 81. The object material P fitted in the rotaryhead 71 is magnetized in the peripheral direction of the object materialP by electromagnet poles 72, 72, the leakage fluxes due to a flaw K aredetected by a sensor 73, and the output signal of the sensor 73 isapplied to the flaw signal analyzer 82. When the output signal levelsupplied from the sensor 73 is not less than a predetermined value, theflaw signal analyzer 82 applies a command to the marking unit 83 andcauses the marking unit 83 to attach a mark along the periphery of acorresponding portion of the object material P. As a result, the objectmaterial P is searched for flaws along the whole periphery and the wholelength thereof.

The conventional apparatus in which an object material is magnetized bya pair of magnet poles, however, poses the problem that the detectionsensitivity is lowered in the case where the direction of a flawoccurring in the object material is approximate to the direction ofmagnetization of the magnet poles.

FIG. 3 is a graph showing the relation between the direction of a flawand the sensor output. The abscissa represents a tilt angle θ withrespect to the center axis of the object material, and the ordinategives the sensor output expressed as a relative value when a flaw occursin the direction of the center axis (i.e., θ=0°). As seen from FIG. 3,the sensor output is strongest when the flaw lies in the direction ofthe center axis of the object material (θ=0°), and steadily decreaseswith the increase in the angle θ. When the angle θ reaches the directionperpendicular to the center axis, i.e., the magnetization direction, thesensor output substantially disappears in spite of a flaw which mayexist.

In view of the above-mentioned fact, an apparatus is proposed asdisclosed in Japanese Patent Application Publication No. 59-22179. Inthis apparatus, two pairs of magnetic poles are arranged in cross inopposed relation with a tubular object material, with each magnetic polebeing connected to an AC power supply. A sensor is provided at eachcrossing point of the two pairs of magnetic poles. The two pairs ofmagnetic poles are excited by AC currents different in phase by π/2supplied from an AC power supply. A rotary magnetic field directedclockwise or counterclockwise is thus formed, and the leakage fluxes dueto a flaw are detected by the sensor located at the center of rotation.As a result, flaws positioned in any direction can be detected with highsensitivity.

In the above-mentioned conventional apparatus, however, a flaw issearched by using a rotary magnetic field generated by application of anAC current. An internal flaw of an object material or a flaw in theinternal surface of a tubular-object material, therefore, cannot besuccessfully detected. Also, since the rotary magnetic field changes indirection with time, it is difficult to detect the direction and depthof a flaw quantitatively. These problems may be obviated bymagnetization using DC current (DC magnetization). In view of the factthat two pairs of magnetic poles are arranged in cross, however,simultaneous DC magnetization by the two pairs of magnetic poles isimpossible. Another related problem is that alternate DC magnetizationswill decrease the flaw detection speed.

The sensor 73 used for detecting leakage fluxes in the above-mentionedleakage flux flaw detection apparatus normally includes a magneticsensor made of a flux-sensing element such as a Hall probe or amagnetoresistive element, or a coil sensor wound on a ferrite core.

Generally, the conventional sensor 73 is such that in order to increasethe detection sensitivity thereof with respect to a linear flaw Koccurring often in the longitudinal direction of the object material P,the cross section of the flux-sensing part is formed in rectangularshape, and the apparatus is arranged in such a manner that thelongitudinal side of the rectangle is parallel to the direction in whicha flaw K extends. The flaw K has a length of 8 mm to 14 mm and is oftensuch a serious flaw as to reduce the product quality. In the sensor 73having a single flux-sensing part, the noises caused by the vibrationsdue to the transportation of the object material P or the rotationaldrive of the rotary head 71 cannot be reduced, resulting in a low S/Nratio. For this reason, a sensor having two flux-sensing parts describedbelow has been realized in practical applications.

FIG. 4 is a plan view showing a model of a conventional sensor fordetecting leakage fluxes. A sensor 76 includes two flux-sensing parts75, 75 in opposed relation to each other. The flux-sensing parts 75, 75are arranged in predetermined spaced relationship with each other alongthe flaw-detection direction indicated by a white arrow in the drawing.The flux-sensing parts 75, 75 are quadrilateral in shape withlongitudinal sides thereof lying in the direction perpendicular to theflaw-detection direction, and have a length L of about 3 mm. Also, thedistance d between the centers 0₁, and 0₂ of the two flux-sensing parts75, 75 is about 1 mm.

FIG. 5 is a plan view showing the manner in which the sensor 76 shown inFIG. 4 searches for a flaw. FIG. 6 is a side sectional view showing thesame situation as FIG. 5. A flaw K often occurs in the longitudinaldirection of the object material P. In order to detect such a flaw Kwith high sensitivity, the sensor 76 searches for a flaw of the objectmaterial P in the direction perpendicular to the longitudinal directionthereof as indicated by a white arrow in FIG. 4 with the flux-sensingparts 75, 75 arranged in parallel to the longitudinal direction of theobject material P.

The object material P is magnetized in a direction different from theflaw-detection direction by the sensor 76, so that as shown in FIG. 6, amagnetic flux J in the vicinity of the flaw K leaks upward of the flaw Kfrom the object material P and returns to the object material P again.As a consequence, when the flux-sensing parts 75, 75 of the sensor 76come to positions on either side of the flaw K, the flux-sensing parts75, 75 detect magnetic fluxes of opposite polarities to each other. Theresulting detection signals of the flux-sensing parts 75, 75 aresubtracted from each other thereby to amplify the flaw signal. The noisegenerated by vibrations due to the transportation of the object materialP or the scanning of the sensor 76, on the other hand, is detected asthe same polarity by the flux-sensing parts 75, 75. The noise signaldetected, therefore, is reduced in level by the above-mentioneddifferential operation, thereby leading to a high S/N ratio.

FIGS. 7A-7C are graphs showing the detection signals of the flux-sensingparts 75, 75 of the sensor 76 in FIG. 6 and the detection signalssubjected to the differential operation. FIG. 7(a) represents adetection signal of a flux-sensing part 75 positioned ahead in theflaw-detection direction, and FIG. 7(b) represents a detection signal ofa flux-sensing part 75 behind the flaw-detection direction, and FIG.7(c) represents a signal subjected to the differential operation. Asseen from FIGS. 7(a) and (b), the flux-sensing part 75 positioned aheadin the flaw-detection direction detects an upward flaw signal followinga downward flaw signal. The flux-sensing part 75 in the behind positiondetects a downward flaw signal at the timing of detection of the upwardflaw signal. On the other hand, an unevenness due to the noise signal isgenerated in the base line of the detection signal of each of theflux-sensing parts 75, 75. The directions of the unevennesses, however,are identical as seen from FIGS. 7(a) and FIG. 7(b). As a result, asseen from FIG. 7(c), when the detection signals of the flux-sensingparts 75, 75 are subjected to the differential operation, the flawsignal is amplified to a large amplitude, and the noise signal isreduced with the base line substantially assuming a linear form. Thusthe S/N ratio is increased and the flaw detection sensitivity isimproved.

The conventional leakage flux sensor described above has the followingproblem. FIG. 8 is a plan view showing the relationship between a sensorhaving two flux-sensing parts and a tilted flaw. In FIG. 8, numeral 76designates a sensor. The sensor 76 for searching for a flaw of theobject material P in the direction of the white arrow, as describedabove, includes flux-sensing parts 75, 75 in the direction perpendicularto the flaw-detection direction. Assuming that a flaw K1 in parallel tothe flux-sensing parts 75, 75 is set as a reference, a flaw K2 indicatedby the dashed line is inclined by θ from the direction of the referenceflaw K1. Assuming that this θ is the tilt angle, generally, theamplitude of the flaw signal detected by the flux-sensing parts 75, 75is at a local maximum when the tilt angle θ is 0° and decreases with theincrease in the tilt angle θ from 0°.

FIG. 9 is a graph showing the relation between the tilt angle θ of aflaw and the amplitude of a flaw signal in the conventional leakage fluxsensor. In this graph, the ordinate represents the amplitude of the flawsignal, and the abscissa the tilt angle θ. As seen from FIG. 9, in theconventional leakage flux sensor, the amplitude of the flaw signalassumes a local maximum when the tilt angle θ is 0° and continues toassume substantially the same level until the tilt angle θ reaches about18°. Beyond this angle, the amplitude of the flaw signal is sharplyreduced. In this case, a tilt angle θ of about 18° is a critical tiltangle.

As shown in FIG. 10, in the conventional leakage flux sensor, the lengthL of the flux-sensing parts 75, 75 is 3 mm and the distance d is 1 mm.The angle θ_(c) (critical tilt angle) formed between a line segmentconnecting the center of an end of a flux-sensing part 75 and the centerof the other end of the other flux-sensing part 75 on the one hand andthe center axis u₁ (u₂) in the longitudinal direction of theflux-sensing part 75 (75) on the other hand is about 18°. As a result,the flaw K having an tilt angle θ of about 18° or more is such that theflaw signal thereof is detected simultaneously by the flux-sensing parts75, 75, and therefore the amplitude of the flaw signal is reduced by thedifferential operation.

In this way, in the conventional leakage flux sensor, the critical tiltangle θ_(c) is so small that it is easily exceeded even for a flaw witha comparatively small tilt angle θ. Thus the amplitude of the flawdetection signal is so small as to cause an erroneous decision.

Even in the case where there exists a flaw having an tilt angle θ withrespect to the reference direction (center axis of the object material),it is thought that such a flaw can be detected with high sensitivity bythe following method. According to this method, as shown in FIG. 11, asingle flux-sensing part 80 having a circular cross section is scannedin the direction perpendicular to the reference direction. In this case,as shown in FIGS. 12(A) and 12(B), regardless of the tilt angle θ of theflaw K, the relative positions of the flux-sensing part 80 and the flawK in the same plane are fixed around the axis of the flux-sensing part80, and therefore the output of the flux-sensing part 80 remainsconstant. As a result, the linear flaw K can be detected with highsensitivity without being affected by the tilt angle θ. In the case ofusing a differential-type sensor having two such flux-sensing parts 80arranged in parallel in the scanning direction and connected to eachother differentially in order to introduce a differential operationeffective for noise signal suppression, however, the same phenomenonoccurs as the two flux-sensing parts 75 having a rectangular crosssection are arranged in parallel as described above. Therefore, anactual flaw inclined in a direction different from the referencedirection cannot be detected with the same sensitivity as a flawextending in the reference direction.

An object of the invention is to provide a method and an apparatus forflaw detection by leakage fluxes in which a flaw can be magnetized by DCcurrent in a different direction without reducing the flaw-detectionspeed.

Another object of the invention is to provide a method and an apparatusfor flaw detection by leakage fluxes in which the tilt angle and depthof a flaw can be detected quantitatively.

A further object of the invention is to provide a method and anapparatus for flaw detection by leakage fluxes in which the leakagefluxes from a flaw can be increased, thereby making it possible todetect a flaw with high sensitivity.

A still further object of the invention is to provide a leakage fluxsensor in which the critical tilt angle can be increased and even a flawwith a comparatively large tilt angle can be detected with highsensitivity.

A yet further object of the invention is to provide a leakage fluxsensor which can be used as a differential-type sensor employing aneffective differential operation low in cost and simple as a noisesuppression means, and in which even an actual flaw tilted in adirection other than the reference direction can be detected with a highaccuracy and with a predetermined sensitivity as far as the depth of theflaw remains the same.

SUMMARY OF THE INVENTION

An apparatus for flaw detection by leakage fluxes according to thepresent invention for magnetizing an object material with a pair ofmagnet poles, detecting leakage fluxes with a sensor arranged betweenthe pair of magnet poles and at the same time searching for a flaw ofthe object material in a predetermined direction, has a pair of magnetpoles for magnetizing the object material in a direction different fromthe flaw-detection direction, another pair of magnet poles arranged inpredetermined spaced relation with the magnetization areas of the twomagnet poles in the flaw-detection direction for magnetizing the objectmaterial in a direction different from the magnetization direction andthe flaw-detection direction, and a plurality of sensors each arrangedbetween each pair of the magnet poles. The two pairs of magnet poleswith the magnetization areas thereof arranged in predetermined spacedrelation with each other in the flaw-detection direction are used formagnetizing the object material in different directions from each otherwith respect to the flaw-detection direction. As a result, a flaw whichmay occur, in whichever direction it is located in the object material,crosses one of the magnetization directions and therefore generatesleakage fluxes. The leakage fluxes thus generated are detected by eitherof the sensors arranged between each pair of the magnet poles. Since thetwo pairs of magnet poles have the magnetization areas thereof arrangedin spaced relation to each other, DC magnetization is possible therebyto detect the direction and depth of a flaw quantitatively.

In the above-mentioned apparatus for flaw detection by leakage fluxes,the magnetization direction of the two pairs of magnet poles can bechanged, and the two sensors have the detection areas thereofchangeable. As a result, the direction in which the object material ismagnetized is adjusted to the tilt angle of a frequently-occurring flawby changing the position of one of the magnet poles constituting eachmagnet pair or by changing the distance between the two magnet poles.This also can increase the leakage fluxes from a flaw. The detectionarea of the sensors, which can be changed appropriately, is adjusted inaccordance with the adjustment of the magnetization direction.

In another apparatus for flaw detection by leakage fluxes according tothe invention, a tubular material is magnetized by a pair of magnetpoles, and leakage fluxes are detected by a sensor arranged between thetwo magnet poles while a flaw of the tubular material is searched for inthe peripheral direction. The two magnet poles are arranged in such amanner as to form two magnetization areas along the peripheral surfaceof the tubular material in directions different from the peripheraldirection, with two sensors each of which is provided in each of the twomagnetization areas. The tubular material is magnetized in twodirections different from the peripheral direction providing theflaw-detection direction by a pair of magnet poles. As a consequence,any flaw which may occur in the tubular material, regardless of thedirection in which it is located, crosses one of the magnetizationdirections, and therefore, leakage fluxes are generated. The leakagefluxes thus generated are detected by the sensors arranged in the twomagnetization areas. A tubular material can be DC magnetized in twodirections by a pair of magnet poles, whereby the direction and depth ofa flaw can be detected quantitatively.

In the above-mentioned apparatus for flaw detection by leakage fluxes,the magnetization direction of the two magnet poles is changeable, andthe two sensors have the detection areas thereof also changeable. As aresult, the direction in which the tubular material is magnetized isadjusted to the tilt angle of a frequently-occurring flaw by changingthe position of one of the two magnet poles or by changing the distancebetween the two magnet poles. The leakage fluxes from the flaw thus canbe increased. Also, the detection area of the sensors, which ischangeable, is appropriately adjusted in accordance with the adjustmentof the magnetization direction.

Still another apparatus for flaw detection by leakage fluxes accordingto the invention has, in addition to the above-mentioned configuration,means for calculating the output ratio between the two sensors, meansfor determining the tilt angle of the target flaw on the basis of thepredetermined relation between the output ratio and the flaw tilt angleand the calculated output ratio, means for correcting the amplitude ofthe output signals of the two sensors on the basis of the predeterminedrelation between the flaw tilt angle and the amplitude change rate ofthe sensor output signal and the tilt angle of the target flawdetermined, and means for calculating the depth of the target flaw onthe basis of the corrected amplitude of the output signal.

A leakage flux sensor according to the invention is arranged in opposedrelation to an object material for detecting leakage fluxes generated ina flaw of a magnetized object material by means of two flux-sensingparts provided in predetermined spaced relationship with each other inthe flaw-detection direction. The two flux-sensing parts are not morethan 4 mm in the distance d between the centers thereof, with the lengthL of not less than 0.5 mm in the direction perpendicular to theflaw-detection direction, and d/L not less than 1. As far as the centerdistance d of the two flux-sensing parts is in the range of not morethan 4 mm, the detection signal is amplified and a flaw can be detectedwith high sensitivity by subtracting the detection signals produced fromthe two flux-sensing parts. The length L of the flux-sensing parts inthe direction perpendicular to the flaw-detection direction can bereduced to a minimum of 0.5 mm. When d/L is not less than 1, thecritical tilt angle θ_(c) is approximately 45° even when the length L is0.5 mm, so that even a flaw with a comparatively large tilt angle θ canbe detected with high sensitivity.

In the above-mentioned leakage flux sensor, assume that the portion ofthe two flux-sensing parts in opposed relation with the object materialis substantially a circle or a regular polygon in shape. Regardless ofthe flaw tilt angle, the levels of the detection signals of the twoflux-sensing parts are substantially the same.

Still another leakage flux sensor according to the invention is a sensorfor detecting magnetic fluxes leaking from a flaw of an object materialmagnetized, and is so configured that the cross section thereof isformed in circle and two flux-sensing parts having different crosssectional areas are arranged concentrically. The concentricalarrangement of two flux-sensing parts with circular cross sectionshaving different cross sectional areas contributes to maintaining aconstant relative position between the two flux-sensing parts and a flawin a horizontal plane around the axis thereof even in the case ofdetecting magnetic fluxes leaking from an actual flaw tilted in adirection other than the reference direction. As a consequence, theoutputs of the flux-sensing parts are always constant and are notaffected by the tilt angle of the flaw. It is thus possible to detectwith high accuracy even an actual flaw tilted in a direction other thanthe reference direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view schematically showing a conventional leakage fluxflaw detection apparatus of rotary type,

FIG. 2 is a partially enlarged view of the model shown in FIG. 1,

FIG. 3 is a graph showing the relation between the flaw tilt angle andthe sensor output,

FIG. 4 is a plan view schematically showing a conventional leakage fluxsensor,

FIG. 5 is a plan view showing the condition in which a flaw is beingsearched for by the sensor shown in FIG. 4,

FIG. 6 is a side view showing the condition in which a flaw is beingsearched for by the sensor shown in FIG. 4,

FIGS. 7A-7C are diagrams showing waveforms of the detection signals ofthe flux-sensing parts of the sensors shown in FIG. 6 and the signalssubjected to differential operation between the two detection signals,

FIG. 8 is a plan view showing the relation between a sensor having twoflux-sensing parts and the tilt of a flaw,

FIG. 9 is a graph showing the relation between the flaw tilt angle andthe amplitude of a flaw signal in a conventional leakage flux sensor,

FIG. 10 is a plan view schematically showing a conventional leakage fluxsensor,

FIG. 11 is a diagram for explaining a conventional method for preventingthe signal amplitude reduction caused by the flaw tilt angle,

FIGS. 12(A) and 12(B) are diagrams for explaining that the reduction ofthe signal amplitude can be prevented by a conventional method usingflux-sensing parts having a circular cross section,

FIG. 13 is a side view schematically showing a leakage flux flawdetection apparatus according to the invention,

FIG. 14 is a partially cutaway perspective view of a rotary head shownin FIG. 13,

FIG. 15 is a sectional view taken in line A--A in FIG. 14,

FIG. 16 is a sectional view taken in line B--B in FIG. 14,

FIG. 17 is a sectional view taken in line C--C in FIG. 14,

FIG. 18 is a plan view schematically showing important parts of aleakage flux flaw detection apparatus according to the invention,

FIG. 19 is a partially cutaway perspective view showing important partsof a leakage flux flaw detection apparatus according to anotherembodiment of the invention,

FIG. 20 is a sectional view taken in line D--D in FIG. 19,

FIG. 21 is a graph showing the relation between the flaw tilt angle andthe outputs of the two sensors,

FIG. 22 is a graph showing the relation between the flaw tilt angle andthe output ratio of the two sensors,

FIG. 23 is a flowchart showing the order of processes in a leakage fluxflaw detection method according to the invention,

FIGS. 24(A) and 24(B) are diagrams showing waveforms representing theoutput signals of the two sensors in the apparatus of FIG. 13 with anobject material having formed therein a plurality of flaws differentonly in tilt angle,

FIG. 25 is a graph showing a predetermined verification curve of theflaw tilt angle,

FIG. 26 is a diagram showing a waveform representing the result ofcorrecting the outputs of the two sensors based on the flaw tilt angle,

FIG. 27 is a plan view schematically showing the condition in which aflaw is being searched for using a first embodiment of a leakage fluxsensor according to the invention,

FIG. 28 is a plan view showing a first embodiment of the leakage fluxsensor,

FIG. 29 is a sectional view taken in line E--E in FIG. 28,

FIG. 30 is a plan view showing a modification of the first embodiment ofthe leakage flux sensor,

FIG. 31 is a plan view showing another modification of the firstembodiment of the leakage flux sensor,

FIG. 32 is a plan view showing an object material,

FIG. 33 is a side view showing an electromagnet pole for magnetizing anobject material,

FIGS. 34A-34D are diagrams showing waveforms of detection signals from aconventional leakage flux sensor and signals obtained by differentialoperation between the two detection signals,

FIGS. 35A-35D are diagrams showing waveforms of detection signals from aleakage flux sensor according to a first embodiment and signals obtainedby differential operation between the two detection signals,

FIG. 36 is a graph showing the relation between the flaw tilt angle andthe amplitude of a flaw signal,

FIGS. 37(A) and (B) are a longitudinal sectional view of a leakage fluxsensor according to a second embodiment of the invention and a sectionalview taken in line F--F in FIG. 37(A), respectively,

FIGS. 38(A) and (B) are diagrams for explaining relative positions of asecond embodiment of a leakage flux sensor and a flaw,

FIG. 39 is a diagram showing waveforms representing detection signalsaccording to a second embodiment of the leakage flux sensor and a signalobtained by differential operation between the two detection signals,

FIG. 40 is a plan view showing a modification of a leakage flux sensoraccording to a second embodiment,

FIGS. 41(A) and 41(B) are a plan view and a side sectional viewrespectively showing another modification of a second embodiment of theleakage flux sensor,

FIG. 42 is a plan view showing an object material,

FIG .43(A) is a diagram showing waveforms of detection signals accordingto a second embodiment of the leakage flux sensor and a detection signalobtained by differential operation between the two detection signals,

FIG. 43(B) is a diagram showing waveforms of a detection signal and adetection signal obtained by differential operation between twodetection signals in a conventional leakage flux sensor, and

FIG. 44 is a graph showing the relation between the flaw tilt angle andthe amplitude of a flaw signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be specifically explained below withreference to the drawings showing embodiments thereof.

(Leakage flux flaw detection apparatus)

FIG. 13 is a side view schematically showing a leakage flux flawdetection apparatus according to the invention. In FIG. 13, P designatesa solid-cylindrical or a tubular object material having ferromagneticproperties. A pair of feeders 23, 23 each having a pair of upper andlower guide rolls for holding and transporting the object material P inthe direction indicated by an arrow are arranged in predetermined spacedrelationship in the transportation range of the object material P. Thefeeders 23, 23 have, interposed therebetween, an annular rotary head 10with a plurality of electromagnet poles for magnetizing the objectmaterial P and a sensor for detecting leakage fluxes. The objectmaterial P is adapted to pass through the rotary head 10.

The rotary head 10 is mounted coaxially with the center axis of anannular rotary unit 20 on the input side thereof for rotating the rotaryhead 10. A flaw of the object material P is searched for over the enterperiphery and the entire length thereof while being transported throughthe rotary head 10 in rotation by feeders 23, 23. The output signal fromthe sensor of the rotary head 10 is applied to a flaw signal analyzer 22through the rotary unit 20. A marking unit 21 for marking the positionof a detected flaw is arranged on the output side of the rotary unit 20.In the case where it is decided that a flaw exists, the flaw signalanalyzer 22 applies a command to the marking unit 21 to mark around acorresponding portion of the object material P.

FIG. 14 is a partially cutaway perspective view of the rotary head 10shown in FIG. 13. FIGS. 15, 16 and 17 are a sectional view taken in lineXV--XV, a sectional view taken in line XVI--XVI, and a sectional viewtaken in line XVII--XVII respectively in FIG. 14. The rotary head 10, asshown in FIGS. 14 and 16, includes a first magnetic pole unit 1 havingtwo electromagnet poles 1a, 1b mounted in opposed relation thereon, adetector 3 having four sensors 3a, 3b, 3c, 3d such as a flux-sensingelement or a search coil mounted thereon for detecting leakage fluxes,and a second magnetic pole unit 2 having two electromagnet poles 2a, 2bmounted in opposed relation in the direction substantially perpendicularto the direction of the electromagnet poles 1a, 1b of the first magneticpole 1, respectively. The sensors 3a, 3b, 3c, 3d are arranged atsubstantially the mid-point of a line connecting the electromagnet poles1a, 1b of the first magnetic pole 1 and the electromagnet poles 2a, 2bof the second magnetic pole 2.

Each of the electromagnet poles 1a, 1b, 2a, 2b of the first magneticpole unit 1 and the second magnetic pole unit 2, as shown in FIGS. 15and 17, have the cores 12, 12, 12, 12 thereof protruding into the rotaryhead 10 by a predetermined length, and magnetizing coils 13, 13, 13, 13are wound on the protrusion. The two electromagnet poles 1a, 1b of thefirst magnetic pole unit 1 are excited to be N pole (S pole), while theelectromagnet poles 2a, 2b of the second magnetic pole unit 2 areexcited in the reverse polarity. As a result, the object material P ismagnetized in four directions at symmetric angles diagonal to the centeraxis thereof.

Although according to the embodiment under consideration, each pair ofthe first electromagnet poles 1a, 1b and the second electromagnet poles2a, 2b are mounted on the first magnetic pole unit 1 and the secondmagnetic pole unit 2 respectively, it is sufficient if one magnet poleis mounted on at least selected one of the first magnetic pole unit 1and the second magnetic pole unit 2 with two magnet poles being mountedin polarities opposite to the magnet pole on the other magnetic pole.Also, in spite of the electromagnet poles being mounted on the rotaryhead 10 for searching a solid-cylindrical or a tubular materialaccording to the present embodiment, a configuration for searching atabular material is applicable with equal effect.

FIG. 18 is a plan view schematically showing important parts of aleakage flux flaw detection apparatus according to the invention. Anelectromagnet pole 1a of a first magnetic pole is arranged upstream ofan object material P transported in the direction indicated by an arrow,and electromagnet poles 2a, 2b of a second magnetic pole downstream inopposed relation with the center axis I of the object material P inbetween. A first sensor 3aand a second sensor 3b are arranged atsubstantially the mid point between one of the first electromagnet poles1a of the first magnetic pole unit 1 and the second electromagnet poles2a, 2b of the second magnetic pole unit 2.

The electromagnet pole unit 1a of the first magnetic pole is excited toN polarity, and the electromagnet poles 2a, 2b of the second magneticpole unit 2 to S polarity, for example. Consequently, there arise amagnetization area of magnetization angle φ in plan view with respect tothe axis perpendicular to the center axis I of the object material P anda magnetization area of magnetization angle -φ symmetric with the firstmagnetization area with respect to the center axis from theelectromagnet pole 1a of the first magnetic pole to the electromagnetpoles 2a, 2b of the second magnetic pole unit 2. The flaw-detectionoperation is performed in the direction of the white arrow so that aflaw K with a tilt angle of θ with respect to the center axis I of theobject material P is magnetized by the above-mentioned magnetizationareas respectively, and the leakage fluxes thereof are detected by thetwo sensors 3a, 3b.

The rotary head 10 shown in FIG. 14 has a spacer interposed between thedetector 3 and the first magnetic pole 1 and also between the detector 3and the second magnetic pole 2, respectively. The spacer is prepared ina variety of widths, whereby the magnetization angle (φ, -φ) can bechanged appropriately. A high detection sensitivity can be maintained bychanging the magnetization angle (φ, -φ) to a frequently-occurring valueof tilt angle φ at predetermined time intervals.

FIG. 19 is a partially cutaway perspective view showing a leakage fluxflaw detection apparatus according to another embodiment of theinvention. A pair of electromagnet poles 1a, 1b are used to magnetizethe object material P in two directions different from theflaw-detection direction. FIG. 20 is a sectional view taken along lineXX--XX in FIG. 19. A groove 4 is formed longitudinally in the innerperipheral wall of a hollow-cylindrical rotary head 10. An electromagnetpole 1a is slidably fitted and fixed at an end of the groove 4. Also, anelectromagnet pole 1b is fixed at a position turned by about 180° withrespect to the electromagnet pole 1a in the inner peripheral wall of theother end of the rotary head 10. In this way, the object material P ismagnetized by the electromagnet poles 1a, 1b in two directions passingthrough the peripheral surface of the object material P connecting thecenters of the electromagnet poles 1a, 1b. Also, the above-mentionedmagnetization angle (φ, -φ) can be changed by changing the position ofthe electromagnet pole 1a. As a result, as in the aforementionedembodiment, a high detection sensitivity can be maintained by changingthe magnetization angle (φ, -φ) to a frequently-occurring tilt angle θ.

Other grooves 6a, 6b are formed in the inner peripheral wall between thetwo electromagnet poles 1a, 1b of the rotary head 10, and support rods5a, 5b for supporting the sensors 3a, 3b are slidably fitted and fixedat the forward end of the grooves 6a, 6b, respectively. The position ofthe support rods 5a, 5b is adjusted so that the sensors 3a, 3b supportedby the support rods 5a, 5b are positioned above the mid point of twoline segments connecting the centers of the two electromagnet poles 1a,1b and passing through the peripheral surface of the object material P.As a result, even when the position of the electromagnet pole 1aischanged, the sensors 3a, 3b are capable of detecting leakage fluxes froma flaw with high sensitivity. Although according to this embodiment theposition of the electromagnet pole 1a is changeable, an arrangement mayof course be made so that the position of the electromagnet pole 1b mayalso be changeable.

FIG. 21 is a graph showing the relation between the flaw tilt angle andthe sensor output of a leakage flux flaw detection apparatus accordingto the invention. In this graph, a solid line represents the output(amplitude) of a sensor (first sensor) described above, and a dashedline the output (amplitude) of the other sensor (second sensor). As seenfrom FIG. 21, the outputs of the two sensors both undergo a change inaccordance with the flaw tilt angle. The changes of the two outputs aresymmetric with each other. The output of the first sensor assumes alocal maximum when the flaw tilt angle θ with respect to an axisorthogonal to the flaw-detection direction is φ, while the output of thesecond sensor takes a local maximum value when the tilt angle θ is -φ.The output ratio of the two sensors is determined as described below.

FIG. 22 is a graph showing the output ratio between the first and secondsensors. As shown in FIG. 22, the output ratio between the first andsecond sensors forms a sigmoid curve passing through a point 1 in thecase where the tilt angle θ is 0, that is, a flaw is located in thedirection perpendicular to the flaw-detection direction, and the outputratio and the tilt angle θ are in one-to-one correspondence to eachother in accordance with this curve. This curve thus can be used as averification curve. In this way, the tilt angle θ of a flaw can becalculated on the basis of the detected output ratio between the firstsensor and the second sensor by determining such a verification curve inadvance.

Once the tilt angle θ of a flaw is calculated, the amplitude of thesensor output signal can be corrected based on the graph of FIG. 21. Thecorrected amplitude of the output signal is directly proportional to theflaw depth, and therefore the flaw depth can be determined bydetermining the relationship between the two factors in advance by atest using an artificial flaw of a known depth.

FIG. 23 is a flowchart showing the sequence of steps for processing theoutput signals of the first sensor 3a and the second sensor 3b shown inFIG. 18. The output signals of the first sensor 3a and the second sensor3b are fetched (step S1) and are converted into digital signals (stepS2). The time difference of detection between the first sensor 3a andthe second sensor 3b for the same flaw is corrected (step S3), followedby calculating the output ratio between the two sensors (step S4).

The tilt angle of a target flaw is determined (step S5) on the basis ofthe relation between a predetermined flaw tilt angle and the outputratio (FIG. 22) on the one hand and the output ratio calculated at stepS4 on the other hand. In similar fashion, the amplitude of the outputsignals of the first sensor 3a and the second sensor 3b is corrected(step S6) on the basis of the relation between a predetermined flaw tiltangle and the change in the output signal amplitude (FIG. 21) on the onehand and the tilt angle of the target flaw determined at step S5 on theother hand. Also, the depth of the target flaw is calculated on thebasis of the relation between a predetermined amplitude of the outputsignal and the flaw depth on the one hand and the corrected amplitude ofthe output signal on the other hand (step S7). Then the tilt angle ofthe target flaw determined at step S5 and the flaw depth determined atstep S7 are applied to an external memory unit or the like (step S8).

Now, explanation will be made about the result of a flaw detection testconducted with a leakage flux flaw detection apparatus according to theinvention. FIGS. 24(A) and 24(B) are diagrams showing waveformsrepresenting the sensor output signals for an object material having aplurality of flaws formed at different tilt angles by using theapparatus shown in FIG. 13 and FIG. 14. The graph of FIG. 24 (A)represents an output signal of the first sensor, and FIG. 24(B)represents an output signal of the second sensor.

The object material serving as a test piece is made of a carbon steeltube on which a notched flaw 20 mm long, 0.5 mm wide and 0.5 mm deep isformed by electrical discharge machining with a tilt angle θ of -50°,-25°, 0°, +25° and +50°. A magneto diode element having a flux-sensingpart with a sectional area of 1 mm×3 mm is used as the first and secondsensors, and electromagnet poles are arranged with the magnetizationangle φ of ±25° to search for a flaw in the test piece.

As a consequence, as shown in FIG. 24, the output of the first sensor(FIG. 24(A)) assumes a local maximum when the flaw tilt angle θ=25° anda local minimum value when the flaw tilt angle θ is -50°. The output ofthe second sensor (FIG. 24(B)), on the other hand, takes a local maximumvalue for the flaw tilt angle θ of -25° and a local minimum value forthe tilt angle θ of 50°. The output ratio between the first and secondsensors then is determined for each tilt angle.

FIG. 25 is a graph showing a verification curve of a predetermined flawtilt angle. The ordinate represents the output ratio, and the abscissathe tilt angle. As a result of determining a flaw tilt angle based onthe output ratio between the first and second sensors and theverification curve described above, the flaw tilt angle formed bymachining is found to coincide well with the tilt angle determined fromthe verification curve.

FIG. 26 is a diagram showing waveforms representing the result ofcorrecting the outputs of the first and second sensors on the basis ofthe flaw tilt angle. The reduction rate of the outputs of the twosensors with respect to the flaw tilt angle is determined in advance,and the output signal in FIG. 24 is corrected on the basis of thereduction rate thus determined. As shown in FIG. 26, the output signalsof the first and second sensors have coincided with each other for allthe tilt angles. Also, since the depth of each flaw formed in the objectmaterial is identical to each other as described above, the amplitudesafter correction are all identical.

(First embodiment of leakage flux sensor)

FIG. 27 is a plan view schematically showing important parts of aleakage flux flaw detection apparatus using the first embodiment of theleakage flux sensor. In FIG. 27, the component elements identical orsimilar to those in FIG. 18 are designated by the same referencenumerals respectively and will not be described any further. The firstsensor 3a and the second sensor 3b each include two solid-cylindricalflux-sensing parts 33a, 33a and 33b, 33b respectively arranged inpredetermined spaced relationship with each other in the flaw-detectiondirection (designated by a white arrow). The leakage fluxes due to aflaw K are detected by the flux-sensing parts 33a, 33a, 33b, 33b of thetwo sensors 3a, 3b respectively. The detection signals of theflux-sensing parts 33a, 33a of the first sensor 3a are applied to a flawsignal analyzer 22 (FIG. 13), where the noise signal is removed bydifferential operation while at the same time amplifying the flawsignal. The detection signals of the flux-sensing parts 33b, 33b of thesecond sensor 3b are also processed in similar fashion. FIG. 28 is aplan view showing a first embodiment of the leakage flux sensor, andFIG. 29 a sectional view taken along line XXX--XXX in FIG. 28. Thesensor 3a (3b), as described above, includes two flux-sensing parts 33,33. The flux-sensing parts 33, 33 have solid-cylindrical cores 34, 34 offerrite arranged in predetermined spaced relationship with each other.Coils 35, 35 are wound in the vicinity of the lower end of the cores 34,34, respectively.

The size of the flux-sensing parts 33, 33 is as follows. The distance dbetween centers 0₁ and 0₂ is not more than 4 mm, the length L of thediameter in the direction perpendicular to the flaw-detection directionis not less than 0.5 mm, and d/L not less than 1. The two flux-sensingparts 33, 33 are arranged in such a manner as to secure the distance din the flaw-detection direction as shown in FIG. 27. The critical tiltangle θ_(c) of the sensor 3a (3b) is 45° to 83°. According to thisembodiment, the flux-sensing parts 33, 33 are provided as a coil sensor.It is needless to say that the flux-sensing parts 33, 33 may be madealternatively of a Hall element or a magnetoresistive element with equaleffect.

FIGS. 30 and 31 are plan views showing modifications of the firstembodiment of a leakage flux sensor. The shape of the flux-sensing parts33, 33 in plan view may be a square as shown FIG. 30 or a polygon suchas a regular hexagon shown in FIG. 31 as well as a circle shown in FIG.28. In this way, the detection signals of the two flux-sensing parts 33,33 are kept at substantially the same level regardless of the flaw tiltangle θ by making the plan view of the flux-sensing parts 33, 33 acircle or a regular polygon.

Now, explanation is made about a test example using a first embodimentof the leakage flux sensor. This test was conducted on a tabularmaterial. FIG. 32 is a plan view showing an object material. The objectmaterial P is made of a rectangular carbon steel sheet 5 mm thick on thesurface of which notched flaws K1, K2 with the thickness of 5 mm, lengthof 20 mm, width of 0.5 mm and depth of 0.5 mm are formed with a tiltangle of 0° or 45° by electrical discharge machining. As shown by thewhite arrow in the drawing, flaws are searched for in the directionperpendicular to the notched flaw K1 with the tilt angle θ of 0°.

FIG. 33 is a side view showing an electromagnet pole unit formagnetizing an object material. In FIG. 33, numeral 31 designates aU-shaped core. The U-shaped core 31 is formed of a plurality of siliconsteel sheets laminated in U shape (outer length of open legs 60 mm×innerlength of open legs 40 mm×height 60 mm×length 65 mm), with the two legswound with coils 32, 32 of copper conductors (outer diameter of 1 mm) in30 turns, respectively. The magnetizing power is 3(A)×60(T)=180(A·T),and the excitation frequency 2 kHz. In order to keep a constant value ofthe magnitude of the leakage fluxes from each of the notched flaws K1,K2 shown in FIG. 32, the notched flaws K1, K2 are magnetized in thedirection perpendicular to the length thereof.

The sensor for detecting leakage fluxes according to this embodimentincludes circular flux-sensing parts as shown in FIG. 28, of which thecenter distance d is 3 mm, the size L in the direction perpendicular tothe flaw-detection direction is 2 mm and the critical tilt angle θ_(c)about 56°. An example of conventional apparatuses, on the other hand,includes rectangular flux-sensing parts as shown in FIG. 10 with thecenter distance d of 3 mm, the size L of 1 mm and the critical tiltangle θ_(c) of about 18°.

FIGS. 34A-34D are graphs showing detection signals by the conventionalsensor, and FIGS. 35A-35D are graphs showing detection signals by asensor according to this embodiment. FIG. 34(a) and FIG. 35(a) representa detection signal of a notched flaw with a tilt angle θ of 0° detectedby one of the flux-sensing parts, FIG. 34(b) and FIG. 35(b) signalsobtained by differential operation between two detection signals of anotched flaw with a tilt angle θ of 0° produced by two flux-sensingparts, FIGS. 34(c) and 35(c) detection signals of a notched flaw with atilt angle θ of 45° produced by one of the flux-sensing parts, and FIGS.34(d) and 35(d) signals obtained by differential operation between twodetection signals of a notched flaw with a tilt angle θ of 45° producedby two flux-sensing parts.

As seen from FIGS. 34(a) and 34(c), in the conventional sensor, the flawsignal with a tilt angle θ of 45° is smaller in amplitude than the flawsignal with a tilt angle θ of 0° so that the detection sensitivityvaries with the flaw tilt angle θ. Also, as obvious from FIGS. 34(b) and34(d), after differential operation between detection signals producedfrom the two flux-sensing parts, the flaw signal with a tilt angle θ of45° exceeds the critical tilt angle θ_(c) and therefore, as comparedwith the flaw signal with a tilt angle θ of 0°, is small in amplitude byabout 70%.

With the sensor according to the present embodiment, as seen from FIGS.35(a) and 35(c), the amplitude of a flaw signal with a tilt angle θ of45° is substantially the same as that of a flaw signal with a tilt angleθ of 0°. Also, it is seen from FIG. 35(b) and 35(d) that differentialoperation between detection signals produced from the two flux-sensingparts leads to a critical tilt angle θ_(c) of about 56° and thereforeamplifies the flaw signal with θ of 45° to the same degree as the flawsignal with a tilt angle θ of 0°.

FIG. 36 is a graph showing the detection result of a notched flaw formedby changing the tilt angle θ at intervals of 10° in the tilt angle rangeof 0° to 90°. In FIG. 36, character c designates the result obtained bythe above-mentioned conventional sensor, and al the result obtained bythe above-mentioned sensor according to this embodiment. Also in FIG.36, a2 and a3 designate the result obtained by sensors according to thisembodiment, of which a2 is the result obtained by a sensor having acenter distance d of 3 mm between two flux-sensing parts, a size L of 2mm in the direction perpendicular to the flaw-detection direction and acritical tilt angle θ_(c) of about 45°, while a3 is the result obtainedby a sensor having a center distance d of 3 mm, a size L of 2 mm and acritical tilt angle θ_(c) of about 83°. The abscissa represents the tiltangle θ, and the ordinate the amplitude of a flaw signal shown with theamplitude of a flaw signal having a tilt angle θ of 0 as 1.

It is seen from FIG. 36 that in the conventional sensor the flaw signalsharply decreases from the tilt angle θ of 20° so that the range of tiltangles that can be accommodated by the conventional sensor is verynarrow. In all the sensors according to the present embodiment, on theother hand, the flaw signal remains substantially the same in magnitudeup to the respective critical tilt angle θ_(c), thereof, with the resultthat the flaw detection requirement can be met down to a tilt angle θ of45° in minimum and up to a tilt angle θ of 83° in maximum.

(Second embodiment of leakage flux sensor)

A leakage flux sensor according to a second embodiment of the inventionis described below. FIG. 37 is a diagram showing an example of theleakage flux sensor according to the second embodiment. FIG. 37(A) is alongitudinal sectional view, and FIG. 37(B) a sectional view taken alongline XXXVII-XXXVII in FIG. 37(A).

As shown in FIGS. 37(A) and 37(B), a leakage flux sensor 41 includes ahollow-cylindrical first flux-sensing part 42 and a solid-cylindricalsecond flux-sensing part 43 provided in the hollow portion of the firstflux-sensing part 42 along the same axis. The first flux-sensing part 42is formed of a conductor 42b (outer diameter of 0.5 mm) of copper or thelike wound (in 20 turns) on the outer periphery of the central portionalong the axis of a hollow-cylindrical ferrite core 42a (outer diameterof 6 mm, inner diameter of 5 mm and a height of 5 mm). The secondflux-sensing part 43, on the other hand, is formed of a conductor 43b(outer diameter of 0.5 mm) of copper or the like wound (in 20 turns) onthe outer periphery of the axial part of a solid-cylindrical ferritecore 43a (axial outer diameter of 2 mm and a height of 5 mm includingthe flange) with a flange 43c (outer diameter of 4 mm and a thickness of0.5 mm) formed at each end thereof.

Assume that a linear flaw K extending in a predicted direction is to besearched for by scanning in a direction perpendicular to the predicteddirection using the leakage flux sensor 41 configured of the firstflux-sensing part 42 and the second flux-sensing part 43 having acircular cross section disposed concentrically. Even in the case wherean actual flaw K is tilted in a direction different from the predicteddirection, the detection sensitivity is not reduced. This is by reasonof the fact that as shown in FIGS. 38(A) and (B), with a flaw K havingthe same shape and the same depth, the relative positions of the firstflux-sensing part 42 and the second flux-sensing part 43 with respect tothe flaw K become constant at the same time in a horizontal plane aroundthe axis thereof, and therefore that the flux-sensing parts 42, 43produce a flaw signal of a predetermined amplitude at the same timecorresponding to the amount of leakage fluxes sensed in accordance withthe size of the cross sectional area of the ferrite cores 42a, 43a,respectively, regardless of the direction in which the flaw K extends.

The output amplitude of noise signals including a loose material signal(e.g., a signal caused by loose material on the surface of the objectmaterial, surface unevenness, or transportation vibrations of an objectmaterial) can be suppressed using the differential connection of the twoflux-sensing parts 42, 43 as in the prior art and the above-mentionedembodiment.

FIG. 39 is a diagram showing an example of a flaw signal waveform of thefirst flux-sensing part 42 and the second flux-sensing part 43configured as described above, and also an example of a flaw signalwaveform of the two flux-sensing parts 42, 43 concentrically arrangedand differentially connected. As seen from FIG. 39, the flaw signalwaveforms for the first flux-sensing part 42 and the second flux-sensingpart 43, both of which have a circular cross section, are seen to have apredetermined amplitude for a linear flaw of a predetermine shape and apredetermined depth, regardless of the direction in which the particularlinear flaw extends. Also, it is seen that with the flaw signalwaveforms obtained when the two flux-sensing parts 42, 43 areconcentrically arranged and differentially connected, the noise signalthereof due to the loose material signal, a gentle change of magneticfield or the like is suppressed.

FIG. 40 is a plan view showing a modification of the leakage flux sensoraccording to the second embodiment. As seen from FIG. 40, the leakageflux sensor 50 in this example includes a first coil 51 large indiameter and a second coil 52 small in diameter concentrically arrangedand printed on a silicon wafer 53 with a conductor 50a of copper or thelike wound spirally in a plurality of turns at predetermined spatialintervals. Numerals 51a, 52a in FIG. 40 designate an input terminal, and51b, 52b an output terminal. It is needless to say that a substantiallysimilar flaw signal waveform to that of FIG. 39 can be obtained with theleakage flux sensor 50 according to this modification which includes thelarge-diameter first coil 51 and the small-diameter second coil 52arranged and printed concentrically on the silicon wafer 53.

FIG. 41 is a diagram showing another modification of the leakage fluxsensor according to the second embodiment. FIG. 41(A) is a plan view,and FIG. 41(B) a side sectional view. As shown in FIG. 41, a leakageflux sensor 60 according to this modification is an example with amagnetic sensor including a flux-sensing element such as a Hall probe ora magnetoresistive element. This sensor includes a first flux-sensingpart 60a and a second flux-sensing part 60b having a circular crosssection with different sectional areas laminated in vertical directionalong the same axis through an insulating layer 60c disposedtherebetween.

In the leakage flux sensor 60 shown in FIG. 41, the first flux-sensingpart 60a and the second flux-sensing part 60b can be formed in reverselayers. Also, as seen from this example, the coil-type leakage fluxsensor 41 shown in FIG. 37 may be configured of a vertical laminationincluding a solid-cylindrical flux-sensing part having a large sectionalarea as the first flux-sensing part 42 and a second flux-sensing part 43of a solid material having a small sectional area. This laminated sensorhas different lift-offs R1, R2 for the flux-sensing parts 60a, 60b withrespect to the surface of the object material P (FIG. 41(B)), resultingin different detection sensitivities. In view of this, calibration ofcourse is required to ensure that signals from the same flaw can bedetected with the same sensitivity.

Now, explanation is made about the result of a test conducted on aleakage flux sensor according to the second embodiment. An objectmaterial is prepared which, as shown in FIG. 42, is made of a 5 mm-thickcarbon steel sheet processed by electrical discharge machining, wherebynotched artificial flaws (20 mm long×0.5 mm wide and 0.5 mm deep) K1 toK7 are formed with counterclockwise tilt angles θ of 0° to 90° with a15° pitch from the direction of extension of the leftmost flaw (K1) as areference. Then, the object material is magnetized under the conditionsof 2 kHz in exciting frequency and 3(A)×60(T)=180(A·T) in magnetizingforce using a magnetizing electromagnet pole of AC type shown in FIG. 33as described above. After that, the flaws K1 to K7 are searched for inthe scanning direction along the white arrow in FIG. 42 using theleakage flux sensor 41 according to the present embodiment which is inthe shape shown in FIG. 37 and has the above-mentioned dimensions. Also,by way of comparison, each flaw is searched for by a similar methodusing a conventional leakage flux sensor configured as shown in FIG. 5and having a rectangular cross section with a long side of 4 mm and ashort side of 1 mm.

FIG. 43 shows detection signal waveforms produced from each flux-sensingpart and detection signal waveforms subjected to the differentialoperation between them for the artificial flaw K1 having a tilt angle θof 0° and the artificial flaw K4 having a tilt angle θ of 45°. FIG.43(A) represents the case using a leakage flux sensor according to thisembodiment, and FIG. 43(B) the case using the conventional leakage fluxsensor. In the latter case handling the prior art, the outputs of thetwo flux-sensing parts are identical, and therefore the outputs of oneof the flux-sensing parts is shown in FIG. 43(B).

As seen from FIGS. 43(A) and 43(B), in the conventional leakage fluxsensor, the amplitude of a flaw signal of the artificial flaw K4 havinga tilt angle θ of 45° is reduced to about 70% of the amplitude of theflaw signal having a tilt angle θ of 0°. With the leakage flux sensoraccording to the present embodiment, in contrast, the amplitude of theflaw signal is not reduced. Also, with either the conventional leakageflux sensor and the leakage flux sensor according to this embodiment,the above-mentioned noise signal is not substantially observed in thesignal waveform after the differential operation although it is observedthat the signal waveform of each flux-sensing part contains a noisesignal caused by a gentle change of magnetic field.

FIG. 44 is a diagram showing the change in detection sensitivity of theleakage flux sensor according to the embodiment and the conventionalleakage flux sensor for the artificial flaws K1 to K7 having a tiltangle θ with the sensor output of 1 for the tilt angle θ of 0°. As seenfrom FIG. 44, the flaw detection sensitivity of the conventional leakageflux sensor is sharply reduced with the increased departure of the flawtilt angle θ away from a predicted direction (reference direction). Incomparison with this, the flaw detection sensitivity of the leakage fluxsensor according to this embodiment is not substantially reduced even inthe case where the flaw tilt angle θ greatly departs from the referencedirection, so that regardless of the flaw tilt angle θ, it is seen thata flaw can be detected with a substantially constant sensitivity.Although the above-mentioned result of a test on the leakage fluxsensors is based on AC excitation, a similar result of course isobtained in the case of DC excitation. Industrial Applicability

As described above, in a leakage flux sensor according to the presentinvention, a flaw occurring in an object material, in whicheverdirection, is detected by a sensor, and therefore DC magnetization ofhigh flaw detection accuracy is possible without reducing the flawdetection speed. As a result, data obtained can be handledquantitatively, and the throughput of the flaw detection work is high.

In the leakage flux flaw detection apparatus according to the invention,leakage fluxes from a flaw can be increased by adjusting the directionin which an object material or a tubular material is magnetized to afrequently-occurring flaw tilt angle, and therefore a flaw can bedetected with high sensitivity.

Further, the leakage flux flaw detection apparatus according to theinvention can quantitatively determine the tilt angle and depth of aflaw formed in whichever direction, and thus can contribute toimprovement of a manufacturing process by tracing the cause of a flawbased on the data obtained.

In a leakage flux sensor according to the invention, the large criticaltilt angle of a flaw permits a flaw detection signal to be amplified tothe same level as when the angle θ is 0 even for a flaw with acomparatively large tilt angle θ, thereby making it possible to avoid anerroneous decision.

Also, the leakage flux sensor according to the invention hassubstantially the same level of detection signals for the twoflux-sensing parts regardless of the flaw tilt angle θ, therebymaintaining a high S/N ratio.

Furthermore, in the leakage flux sensor according to the inventionincluding two concentrically-arranged sensors having a circular crosssection of different sectional areas, a linear flaw can be detectedalways with a constant sensitivity regardless of the direction in whicha flaw occurs in the case where the direction of magnetization is atright angles to the direction in which the linear flaw extends. Also,when each flux-sensing part is used in differential connection, thenoise signals such as a loose material signal and a gentle change ofmagnetic field can be successfully suppressed.

I claim:
 1. A method for flaw detection by leakage fluxes in which anobject material is magnetized by a pair of magnet poles and leakagefluxes are detected by a sensor interposed between said pair of magnetpoles while searching for a flaw of the object material in apredetermined flaw-detection direction, comprising the stepsof:magnetizing the object material by a pair of magnet poles in adirection different from the flaw-detection direction; magnetizing theobject material by another pair of magnet poles in a direction differentfrom the magnetization direction and flaw-detection direction inpredetermined spaced relation from the magnetization area of said twomagnet poles in the flaw-detection direction; detecting the leakagefluxes from the object material by each of sensors interposed betweeneach of the two pairs of magnet poles; and identifying a flaw of theobject material on the basis of the detection result of leakage fluxeswherein the step of identifying a flaw includes the steps of:calculatingthe output ratio between said two sensors; determining the tilt angle ofa flaw on the basis of a predetermined relationship between the outputratio and the flaw tilt angle on the one hand and the calculated outputratio on the other hand; correcting the amplitude of the outputs of saidtwo sensors on the basis of a predetermined relationship between theflaw tilt angle and the amplitude change rate of the sensor outputsignal on the one hand and the tilt angle of the target flaw determinedon the other hand; and calculating the depth of the flaw on the basis ofthe corrected amplitude of the outputs.
 2. An apparatus for flawdetection by leakage fluxes in which an object material is magnetized bya pair of magnet poles and leakage fluxes are detected by a sensorinterposed between said two magnet poles while searching for a flaw ofthe object material in a predetermined flaw-detection direction,comprising:a first pair of magnet poles positioned for magnetizing theobject material in a direction different from the flaw-detectiondirection; a second pair of magnet poles for magnetizing the objectmaterial in a direction different from the magnetization direction andflaw-detection direction in predetermined spaced relation from themagnetization area of said two magnet poles in the flaw-detectiondirection; two sensors each of which is disposed between each of saidfirst and second pairs of magnet poles for sensing magnetic flux; meansfor calculating the output ratio between said two sensors; means fordetermining the tilt angle of the target flaw on the basis of apredetermined relationship between the output ratio and the flaw tiltangle on the one hand and the calculated output ratio on the other hand;means for correcting the amplitude of the output signals of said twosensors on the basis of a predetermined relationship between the flawtilt angle and the amplitude change rate of the sensor output signal onthe one hand and the tilt angle of the flaw determined on the otherhand; and means for calculating the depth of the flaw on the basis ofthe corrected amplitude of the output signals.
 3. An apparatus for flawdetection by leakage fluxes in which an object material is magnetized bya pair of magnet poles and leakage fluxes are detected by a sensorinterposed between said two magnet poles while searching for a flaw ofthe object material in a predetermined flaw-detection directioncomprising:a first pair of magnet poles positioned for magnetizing theobject material in a direction different from the flaw-detectiondirection; a second pair of magnet poles for magnetizing the objectmaterial in a direction different from the magnetization direction andflaw-detection direction in predetermined spaced relation from themagnetization area of said two magnet poles in the flaw-detectiondirection; two sensors each of which is disposed between each of saidfirst and second pairs of magnet poles for sensing magnetic flux whereinsaid first and second pairs of magnet poles have changeable directionsof magnetization and said two sensors have changeable detection areas;means for calculating the output ratio between said two sensors; meansfor determining the tilt angle of the target flaw on the basis of apredetermined relationship between the output ratio and the flaw tiltangle on the one hand and the calculated output ratio on the other hand;means for correcting the amplitude of the output signals of said twosensors on the basis of a predetermined relationship between the flawtilt angle and the amplitude change rate of the sensor output signal onthe one hand and the tilt angle of the flaw determined on the otherhand; and means for calculating the depth of the flaw on the basis ofthe corrected amplitude of the output signals.